In order to remove SO2 from gaseous streams, it is known to utilize a process with regenerable aqueous amine absorbents. The absorbent is exposed to a gas stream to absorb SO2 from the gas stream and to produce an SO2 lean treated gas stream and a spent absorbent containing amine SO2 salts. The amine SO2 salts in the spent absorbent are decomposed in the regenerator tower of the process to gaseous SO2 and the corresponding free base amine under elevated temperature and stripping steam traveling up the regeneration column, countercurrent to the spent absorbent flowing downwards.
However, in commercial operation, acid gas capture processes experience ingress and/or generation in process of acids that are stronger than the acids for which the removal process is designed. These stronger acids form salts with the amine solvent which are not regenerable with steam and are termed heat stable amine salts (HSAS), or heat stable salts (HSS). If sodium cations enter the solvent either through intentional addition or unintentional means such as carryover in a mist, the sodium, being a stronger base than the amine, will remove anions from the amine and form HSS.
If the heat stable amine salts are allowed to accumulate, they will eventually neutralize all the amine of the solvent, rendering it unable to react with and remove the acid gas component as intended. Accumulation of sodium salts can eventually reach their solubility limit, causing undesirable precipitation of solids in the process. Therefore, as it is known in the art, means for removal of heat stable amine salts are either installed as a part of the process or available on demand.
Various means and processes for removal of heat stable salts from amine gas treating solutions are known. These include distillation of the free amine away from the salt at either atmospheric or subatmospheric pressure (see for example “Gas Purification”, 5th edition, Arthur Khol, Gulf Publishing Co, 1997, pages 255 to 263). The disadvantage of distillation is that it is very energy consuming. Other methods include electrodialysis, as described in for example U.S. Pat. No. 5,292,407, and ion exchange. Ion exchange is for example described in U.S. Pat. No. 6,245,128. It describes the reclamation of spent aqueous alkanolamine solutions by contacting the spent solution with a strong base ion exchange resin. After a high concentration of ions accumulate on the resin, the strong base ion resin needs to be regenerated by purging the resin with water, followed by contacting the resin with a sodium chloride solution for a time sufficient to remove the ions. Then the resin is again purged, followed by contacting the resin with a solution with sodium hydroxide to convert the resin to the hydroxide form and again purging the resin. Other examples of processes for the removal of heat stable salts can be found in for example U.S. Pat. No. 4,122,149; U.S. Pat. No. 4,113,849; U.S. Pat. No. 4,970,344; U.S. Pat. No. 5,045,291; U.S. Pat. No. 5,292,407 and U.S. Pat. No. 5,368,818. One of the disadvantages of removal of heat stable salts via electrodyalysis or via ion exchange is that significant amounts of liquid waste are being generated. Another disadvantage is that the losses of process amines are significant.
In GB-A-1118687 a process is described for the recovery of amines from amine degradation products, wherein the solution comprising the amine degradation products are being contacted with an alkaline compound, with such an amount that phase separation into a purified amine-rich phase and an aqueous phase comprising potassium salt is affected. The process is preferably performed at temperatures between 100-125° C. to free certain amine complexes formed. According to GB-A-1118687 a further advantage of temperatures between 100-125° C. is that in practice amine solutions generally have a temperature in this range after the regeneration step in which amine salts of acidic gases are decomposed. The regenerated solutions need not be cooled or heated to apply the process.
A disadvantage of GB-A-1118687 is that distillation is required to reduce the water content of the amine solution before the reaction with the alkaline compound can take place.
Although progress has been made in the regeneration of absorbents by the removal of heat stable salts, there still remains a need for a process that allows higher concentrations of heat stable salts to be removed.